This invention is related to cellular telephones and, in particular, to cellular telephones equipped with the capability of determining their location using the Global Positioning Satellite (GPS) system.
It is required for cellular telephone operation for a cellular phone to be aware of what cell it is located within. Cells in the cellular telephone system refer to the area covered by singular cellular tower, and can overlap. While the area in a cell is covered by a single tower, a tower may support multiple cells when sectored antennae are used at the tower. In general, a cellular phone will monitor the control channel of the cellular tower having the strongest signal. Therefore, a cellular phone has a rudimentary knowledge of its location, but not necessarily in terms of latitude, longitude and altitude.
However, there are applications, other than the normal voice communications capabilities of the cellular phone, that may require a more exact knowledge of the phone""s location within a cell. These include, for example, an application providing location sensitive advertising over the cell phone, wherein targeted advertising could be sent to cell phone users within a certain proximity of the advertiser""s location. Other such applications would be the provision of travel directions based on current location and enhanced 911 emergency services from calls made using a cellular phone. As a result, it is known to combine a cellular phone with GPS receiver in a single unit and to allow applications to request that the GPS receiver provide the current position of the unit to the applications, either periodically or upon request of the application.
To improve certain characteristics of its performance, such as time-to-first-fix and sensitivity, it is possible to provide the GPS receiver with various aiding information, such as satellite ephemeris, approximate time and reference location. The reference location is an approximation or estimate of the receiver""s current position, which, when used with other information, allows the receiver to determine which satellites should be visible and to estimate the signal transit times from the respective satellites. Generally, without this aiding information, acquiring the satellite signals and computing the receiver""s exact location could take much longer. This delay could have a serious impact on the performance of mobile location-based services, which tend to be time-sensitive.
Further, it is possible that there are certain locations, such as when the receiver is indoors, where the signal strength from the GPS satellites is not strong enough to provide a location fix. In these circumstances, the reference location could provide a rough estimate of location.
It is therefore desirable to provide a means by which a reference location can be provided to a GPS receiver which is integrated into a cellular phone.
The invention provides a means whereby the reference location for the GPS receiver can be computed to a higher degree of accuracy using information provided over the cellular network, such as the location of the cellular tower, the current positions of the GPS satellites, and the GPS time reference, and also using positions previously determined using actual fixes from the GPS receiver.
In particular, at selected times when a GPS equipped cell phone is operating in idle (non-call) mode receiving a control channel being broadcast on a particular cellular system, the cell phone uses the GPS receiver to compute its own position. These position computations are associated with the control channel to obtain an approximate location for the cell. The positions are also associated with the neighboring control channels that the cell phone is able to receive and with the signal strengths of the received signals. This allows the cell phone to increase the accuracy of reference locations and differentiate between locations within a cell by using xe2x80x9cneighbor listsxe2x80x9d. The accuracy is thereby increased beyond the granularity of the cell but still is not as accurate as the instantaneous position that may be computed by the GPS receiver when an actual position is needed. The accuracy, however, aids the receiver in more quickly computing the precise locations when needed.
The invention also uses the GPS receiver to compute a position when a private system not broadcasting a reference location is detected by the cell phone. Private systems are often located in an indoor environment, where signals from the GPS satellites may be undetectable. When the cell phone detects that it is in the vicinity of a private system, the the cell phone tracks its position until the signals from the GPS satellites are no longer detectable. The last recorded position is then used as a reference location estimate for applications when the cell phone is indoor and in contact with the private system.
The invention also allows the cell phone to form an estimate of the size of the cell which helps to determine if the reference location of the cell is accurate enough for a particular application. Thus, reducing the number of times the GPS receiver needs to be used.
The message described in this invention may therefore be used to first increase the performance of location estimation when a reference location is unavailable and second, to increase the accuracy of a provided reference location without having to use a GPS receiver in real time to obtain the location fix. The GPS receiver is used to obtain location information for a cell during a prior learning phase. The information obtained is stored in the memory of the cell phone and may be later used for higher accuracy location estimates without having to turn on the GPS receiver to get a real time location fix.